Automatic License Plate Reading (ALPR) systems are used by security and law enforcement personnel to find and read vehicle license plate numbers in images produced by video cameras and still cameras. As shown in FIG. 4, a typical ALPR system consists of a video imaging device, a video capture device and a computer running ALPR software. The video camera sends video images to the video capture device as a standard television format video signal. The video capture device converts the video signal into digital image information and stores the digital information in the computer. The computer's software then begins the ALPR process by first locating the license plate in the image as represented by the digital information. If a license plate is detected in the image, the computer software performs a character recognition process to “read” the license plate number.
In a typical ALPR system, video images are sent to the video capture device at either 50 or 60 images (or frames) per second, depending on the video format. In typical applications of ALPR systems, only a small fraction of the images received by the video capture device actually contain a license plate. Since the prior systems provided no indication to the computer regarding which images contained a license plate, the computer's software had to process each and every image to determine whether a license plate appeared in the image. Since computer-implemented license plate detection algorithms tend to be very computationally intense, a large portion of the computer's processing power in prior systems was used to search for and detect the presence of a license plate.
What is needed, therefore, is a means for flagging the video images that may include a license plate, and for processing only those flagged images to read the license plate character information.